1. Field of the Invention
The invention relates to a method for image processing a digital color image to obtain a print signal, that indicates for each pixel whether a dot of marking material of a corresponding process color is to be applied. The invention further relates to a computer readable medium comprising computer executable instructions and to a color printer system comprising a controller for scheduling and interpreting print jobs and a print engine for marking support material.
2. Description of the Related Art
Digital color images exist in various forms and originate from different sources. They exist in the form of descriptive elements in a page description language (PDL), generated by a computer application. They also exist in the form of a pixel map, in which the image is divided into pixels that carry specifications of the local color of the image, such as images originating from scanners or digital cameras. In order to print these images, a color printer is used with a number of process colors in the form of marking material, that is applied to a support material, such as paper, that receives the image. In most printers, these process colors comprise black, cyan, magenta, and yellow marking material.
Before a print process can be started, a digital separation image is derived for each process color from the digital color image. The digital separation image specifies for each pixel of an image to be printed, a color density value that is representative for the amount of marking material that is to be applied for that pixel. The color density value can be expressed as a percentage between 0% and 100%, wherein a value of 0% for an area with several pixels leads to a minimum amount of marking material for that area and a value of 100% for an area with several pixels leads to a maximum amount of marking material for that area. The color of each pixel is thus represented by a combination of process colors.
A print process is often only capable of printing a fixed amount of marking material on a print position or no marking material at all. A print position is a part of the image on the support material and the fixed amount of marking material is a dot that can be placed on this position. In an image processing step commonly referred to as halftoning, the digital separation image for each process color is converted into a print signal that for each print position has one of two possible values indicating whether or not to apply a dot. This is done in a way that keeps the average marking material density around the print position equal to the color density value of the associated pixel. A well-known way to apply halftoning is dithering, in which a threshold value is associated with each print position. The color density value of a pixel associated to the print position is compared to this threshold value to obtain a value of the print signal. For color density values larger than the associated threshold value, a dot is applied, for color density values smaller or equal to the associated threshold value, no dot is applied. An array of threshold values is associated with each process color to derive a print signal that indicates the dots of marking material.
The distribution of the threshold values over the various print positions determine the dot patterns that occur for various color density values. An array of threshold values is repeatedly applied to associate every part of the image with a threshold value. The oldest dithering techniques use a different array of threshold values for each process color. This causes independent dot patterns for different colors with the effect that a difference in registration of one of the colors does not effect the color impression of the combined dot patterns. In arrays of threshold values for dispersed dithering, the threshold values are distributed in such a way that when a dot pattern is extended with a next dot, this next dot is placed remote from the already existing dots. However, the position of a next dot of another color is uncorrelated with the already existing dots of the first color, because a different array of thresholds is used. This causes some dots of two process colors to be placed on top of each other, while also print positions exist that receive no dot at all. The resulting print quality of the image shows a certain graininess by this.
In patent specifications EP 1,300,000 and U.S. Pat. No. 6,250,733, the dot positions of several colors are derived from only one array of threshold values. In that way, no dots will be put onto each other before all print positions have received a dot. When all print positions receive a dot, a next dot will be placed again on the first print positions, i.e. print positions that are used for dots at very low densities, making those print positions the first ones that receive two dots onto each other. To improve the registration latitude, the patent specification U.S. Pat. No. 7,095,530 already starts using the first print positions of the only array of threshold values before all print positions of the support material have received a dot.
In patent specification EP1,443,748, one threshold array is used until all print positions have received a dot and in case more dots are defined, another threshold array is used, which is uncorrelated with the first one. This prevents image artifacts, such as Moire and other undesired interference patterns for certain combinations of color densities. However, a problem still exists in the balance between graininess in the image and the sensitivity for misregistration between various process colors. Therefore, an object of the invention is to provide a method that improves this balance between these print quality aspects.